The present invention relates to a toner for non-magnetic one-component development, which is used for facsimile, copying machine, laser printer, etc. More particularly, it relates to a toner for non-magnetic one-component development, which is suitable for using in combination with a single-layer type organic photoconductor, and a method for contact type development using the same.
In copying machines using an electrophotographic system, a simple method for one-component insulating toner development, particularly method for non-magnetic one-component contact type development (impression development) has recently been proposed in place of a method for two-component magnetic brush development, and the research and development thereof has been making progress.
The method for non-magnetic one-component contact type development is a kind of a method for reversal development. In this method, as shown in FIG. 1, a toner 1 being a non-magnetic one-component developer is charged by friction charging with a developing roller 2, and then adhered on the surface of the developing roller 2 by an action of a control blade 3 and an image force to form a homogeneous thin film of the toner particle on the developing roller 2. Thereafter, this thin film is contacted directly with an electrostatic latent image formed on a photoconductor drum 4 to actualize the electrostatic latent image as a visual image. The toner 1 is supplied from a toner hopper 5, and then supplied to the developing roller 2 through a toner supply roller 7 while agitating with a toner agitator 6.
The toner for non-magnetic one-component development to be used for such a method for non-magnetic one-component contact type development is formed by optionally adding a surface treating agent such as hydrophobic silica particle to a toner particle wherein a colorant such as carbon black is contained in a fixing resin.
On the other hand, an organic photoconductor (OPC) is used in place of a conventional inorganic photoconductor using amorphous selenium, amorphous silicon, etc. for the photoconductor drum, according to a request to remove environmental pollution. The organic photoconductor is obtained by dispersing a photoconductive polymer or lower molecular compound in a binding resin, and included a so-called function separating type organic photoconductor comprising an electric charge generating layer and an electric charge transferring layer, which are mutually laminated, i.e. multi-layer type photoconductor, and a single-layer type photoconductor comprising an electric charge generating material and an electric charge transferring material, which are contained in a single photosensitive layer.
A conventional toner to be used for the method for non-magnetic one-component contact type development had a problem in build up of charging. Furthermore, the residual potential of the organic photoconductor is high in general, and the potential width which can be used for developing is narrow. Accordingly, high level has hitherto been requested to the charging stability of the toner.
As a result, the charged amount of the toner supplied to the developing roller after the black solid part was developed becomes low. Therefore, there was a problem that the residual image of the black solid part is remained at the half tone part when the half tone is developed immediately after the black solid development, which results in excessively black half tone part. To remain the above residual image is also referred to as "leaving trail."
That is, FIG. 2 is a developing sensitivity curve illustrating a relation between the developing bias (i.e. effective potential difference which is a difference between the potential of the surface of the photoconductor and actual developing bias) and image density, and a solid line shows a curve at the normal state. The black solid part is developed when the developing bias is (a), i.e., normally about +300 to +200 V. The half tone part is developed when the developing bias is (b), i.e., normally about +100 V. At this point, the image density of the half tone part is represented by (I). Incidentally, the white part is obtained when the developing bias is (c), i.e., normally about -400 V. To the contrary, when the amount of the charged amount of the toner becomes small, a rapid build up is observed as shown by the chain line. Therefore, there is a problem that the image density becomes (I+.alpha.) when the half tone part is developed at the same developing bias (b) and the image density becomes high by the amount of .alpha.. It is considered that such a problem arises because the charged amount of the toner is low, thereby decreasing a force of returning the toner to the developing roller by the electric field. This tendency becomes more remarkable when using an organic photoconductor having a high residual potential, particularly single-layer type organic photoconductor.